A solar cell converts light energy to electrical energy by the use of semiconductor properties.
If briefly explaining structure and principle of the solar cell, the solar cell has a PN junction structure by junction of positive(P) type semiconductor and negative(N) type semiconductor. When solar light is incident into the solar cell, hole and electron are generated in the solar cell owing to energy of the incident solar light. Due to an electric field formed in the PN junction, the hole drifts to the P type semiconductor, and the electron drifts to the N type semiconductor, to thereby generate an electric potential.
The solar cell may be classified into a bulk solar cell and a thin film solar cell.
The bulk solar cell may be manufactured by using a substrate of semiconductor material such as silicon, and the thin film solar cell may be manufactured by forming a thin-film semiconductor layer on a glass substrate.
The thin film solar cell may be classified into Si thin film solar cell and compound thin film solar cell on the basis of material for a light-absorbing layer. Also, the compound thin film solar cell may be classified into III-V solar cell and CIGS solar cell.
The CIGS solar cell uses a light-absorbing layer which is formed by a compound of copper (Cu), indium (In), gallium (Ga) and selenium (Se).
Hereinafter, a related art CIGS solar cell will be described with reference to the accompanying drawings.
FIG. 1 is a cross sectional view illustrating the related art CIGS solar cell.
As shown in FIG. 1, the related art CIGS solar cell may include a substrate 1, a rear electrode 2, a light-absorbing layer 3, a buffer layer 4 and a front electrode 5.
The rear electrode 2 is formed on the substrate 1. Generally, the rear electrode 2 is made of molybdenum (Mo). The light-absorbing layer 3 is formed on the rear electrode 2, and the light-absorbing layer 3 is made of a compound of copper (Cu), indium (In), gallium (Ga) and selenium (Se). The buffer layer 4 is formed on the light-absorbing layer 3. Generally, the buffer layer 4 is made of cadmium sulfide (CdS). The front electrode 5 is formed on the buffer layer 4, and the front electrode 5 is made of transparent conductive oxide (TCO).
The related art CIGS solar cell may be manufactured by forming the rear electrode 2 of molybdenum (Mo) on the substrate 1, forming a multi-layered precursor layer including a first precursor layer of copper-gallium (CuGa) and a second precursor layer of indium (In) on the rear electrode 2, forming the light-absorbing layer 3 of CIGS by a thermal treatment under Se atmosphere, forming the buffer layer 4 of cadmium sulfide (CdS) on the light-absorbing layer 3, and forming the front electrode 5 of transparent conductive oxide (TCO) on the buffer layer 4.
A point of the CIGS solar cell is the light-absorbing layer 3 of CIGS. In order to improve efficiency and yield of solar cell, it is necessary to find an optimal method for forming the light-absorbing layer 3. Especially, as mentioned above, the light-absorbing layer 3 is formed by depositing the precursor layer, and performing the high-temperature thermal treatment thereto. Thus, it is very important to improve efficiency of the thermal treatment.
However, a related art system for the thermal treatment has limitations on the thermal treatment for the precursor layer.
Especially, there has been proposed Korean Patent Publication No. P2011-0121443 which relates to a thermal treatment system for a large-sized substrate. However, a temperature distribution in an entire reaction space is not uniform, and a concentration distribution of reaction gas is not uniform for a thermal treatment process.